Image sensors with silver-nanoparticle electrodes

ABSTRACT

Disclosed herein is an apparatus comprising: an array of avalanche photodiodes (APDs) or an absorption region comprising a semiconductor single crystal such as a CdZnTe single crystal or a CdTe single crystal. The apparatus may be configured to absorb radiation particles incident on an absorption region of the APDs or the semiconductor single crystal and to generate charge carriers. The apparatus may comprise an electrode comprising silver nanoparticles and being electrically connected to the absorption region of the APDs or the semiconductor single crystal. For the APDs, each of the APDs may comprise an amplification region, which may comprise a junction with an electric field in the junction. The electric field may be at a value sufficient to cause an avalanche of charge carriers entering the amplification region, but not sufficient to make the avalanche self-sustaining. The junctions of the APDs may be discrete.

TECHNICAL FIELD

The disclosure herein relates to image sensors, particularly relates toimage sensors with silver-nanoparticle electrodes.

BACKGROUND

An image sensor or imaging sensor is a sensor that can detect a spatialintensity distribution of a radiation. The radiation may be anelectromagnetic radiation such as infrared light, visible light,ultraviolet light, X-ray or γ-ray. The radiation may be of other typessuch as α-rays and β-rays. The radiation may be one that has interactedwith a subject. For example, the radiation measured by an image sensormay be a radiation that has penetrated or reflected from the subject.

An image sensor usually represents the detected image by electricalsignals. Image sensors based on semiconductor devices may be classifiedinto several types, including semiconductor charge-coupled devices(CCD), complementary metal-oxide-semiconductor (CMOS), N-typemetal-oxide-semiconductor (NMOS). A CMOS image sensor is a type ofactive pixel sensor made using the CMOS semiconductor process. Lightincident on a pixel in the CMOS image sensor is converted into anelectric voltage. The electric voltage is digitized into a discretevalue that represents the intensity of the light incident on that pixel.An active-pixel sensor (APS) is an image sensor that includes pixelswith a photodetector and an active amplifier. A CCD image sensorincludes a capacitor in a pixel. When light incidents on the pixel, thelight generates electrical charges and the charges are stored on thecapacitor. The stored charges are converted to an electric voltage andthe electrical voltage is digitized into a discrete value thatrepresents the intensity of the light incident on that pixel.

SUMMARY

Disclosed herein is an apparatus comprising: an array of avalanchephotodiodes (APDs), each of the APDs comprising an absorption region, anelectrode and a first amplification region; wherein the absorptionregion is configured to generate charge carriers from a photon absorbedby the absorption region; wherein the electrode comprises silvernanoparticles and is electrically connected to the absorption region;wherein the first amplification region comprises a junction with anelectric field in the junction; wherein the electric field is at a valuesufficient to cause an avalanche of charge carriers entering the firstamplification region, but not sufficient to make the avalancheself-sustaining; wherein the junctions of the APDs are discrete.

According to an embodiment, each of the APDs further comprises a secondamplification region between the absorption region and the electrode,wherein the first amplification region and the second amplificationregion are on opposite sides of the absorption region.

According to an embodiment, the silver nanoparticles comprise silvernanowires.

According to an embodiment, a number density of the silver nanoparticlesis above an electrical percolation threshold of the silvernanoparticles.

According to an embodiment, the electrode further comprises a conductivepad in electrical contact with a portion of the silver nanoparticles.

According to an embodiment, the electrode is a common electrode sharedby the absorption regions of the array of APDs.

According to an embodiment, the electrode further comprises a coatinglayer on the silver nanoparticles.

According to an embodiment, the photon is a soft X-ray photon.

According to an embodiment, the absorption region has a thickness of 10microns or above.

According to an embodiment, the absorption region comprises silicon.

According to an embodiment, an electric field in the absorption regionis not high enough to cause avalanche effect in the absorption region.

According to an embodiment, the absorption region is an intrinsicsemiconductor or a semiconductor with a doping level less than 10¹²dopants/cm³.

According to an embodiment, the absorption regions of at least some ofthe APDs are joined together.

According to an embodiment, the first amplification regions of the APDsare discrete.

According to an embodiment, the junction is a p-n junction or aheterojunction.

According to an embodiment, the junction comprises a first layer and asecond layer, wherein the first layer is a doped semiconductor and thesecond layer is a heavily doped semiconductor.

According to an embodiment, the first layer has a doping level of 10¹³to 10¹⁷ dopants/cm³.

According to an embodiment, the first layers of least some of the APDsare joined together.

According to an embodiment, the junction is separated from a junction ofa neighbor junction by a material of the absorption region, a materialof the first or second layer, an insulator material, or a guard ring ofa doped semiconductor.

According to an embodiment, the guard ring is a doped semiconductor of asame doping type as the second layer and the guard ring is not heavilydoped.

According to an embodiment, the junction further comprises a third layersandwiched between the first and second layers; wherein the third layercomprises an intrinsic semiconductor.

According to an embodiment, the third layers of at least some of theAPDs are joined together.

Disclosed herein is an apparatus comprising: a substrate; asemiconductor single crystal in a recess in the substrate; an electrodeon the semiconductor single crystal; wherein the apparatus is configuredto absorb radiation particles incident on the semiconductor singlecrystal and to generate charge carriers; wherein the electrode comprisessilver nanoparticles and is electrically connected to the semiconductorsingle crystal.

According to an embodiment, the silver nanoparticles are silvernanowires.

According to an embodiment, a number density of the silver nanoparticlesis above an electrical percolation threshold of the silvernanoparticles.

According to an embodiment, the electrode further comprises a conductivepad in electrical contact with a portion of the silver nanoparticles.

According to an embodiment, the electrode further comprises a coatinglayer on the silver nanoparticles.

According to an embodiment, the semiconductor single crystal is a CdZnTesingle crystal or a CdTe single crystal.

According to an embodiment, the substrate comprises silicon, germanium,GaAs or a combination thereof.

According to an embodiment, a surface of the semiconductor singlecrystal and a surface of the substrate are coextensive.

According to an embodiment, the apparatus further comprises anotherelectrode in electrical contact with the semiconductor single crystal;an electronics layer bonded to the substrate, the electronics layercomprising an electronic system configured to process an electricalsignal generated from the charge carriers collected by the otherelectrode.

According to an embodiment, the electronic system comprises a voltagecomparator configured to compare a voltage of the electrode to a firstthreshold; a counter configured to register a number of radiationparticles absorbed by the substrate; a controller; a voltmeter; whereinthe controller is configured to start a time delay from a time at whichthe voltage comparator determines that an absolute value of the voltageequals or exceeds an absolute value of the first threshold; wherein thecontroller is configured to cause the voltmeter to measure the voltageupon expiration of the time delay; wherein the controller is configuredto determine a number of radiation particles by dividing the voltagemeasured by the voltmeter by a voltage that a single radiation particlewould have caused on the other electrode; wherein the controller isconfigured to cause the number registered by the counter to increase bythe number of radiation particles.

Disclosed herein is a system comprising any of the above apparatuses andan X-ray source, wherein the system is configured such that theapparatus forms an image of an object using X-ray from the X-ray sourcethat penetrated the object.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 schematically shows the electric current in an APD as a functionof the intensity of light incident on the APD when the APD is in thelinear mode, and a function of the intensity of light incident on theAPD when the APD is in the Geiger mode.

FIG. 2A, FIG. 2B and FIG. 2C schematically show the operation of an APD,according to an embodiment.

FIG. 3A schematically shows a cross section of an image sensor based onan array of APDs, according to an embodiment.

FIG. 3B and FIG. 3C schematically shows a perspective view of theelectrode comprising silver nanoparticles, according to an embodiment.

FIG. 3D shows a variant of the image sensor of FIG. 3A.

FIG. 3E shows a variant of the image sensor of FIG. 3A.

FIG. 3F shows a variant of the image sensor of FIG. 3A.

FIG. 4A schematically shows a detailed cross-sectional view of an imagesensor, according to an embodiment.

FIG. 4B schematically shows a top view of the radiation absorption layerin FIG. 4A, according to an embodiment.

FIG. 5 schematically illustrates a process of forming an electrode,according to an embodiment.

FIG. 6 schematically shows a system comprising the image sensordescribed herein.

FIG. 7 schematically shows an X-ray computed tomography (X-ray CT)system.

FIG. 8 schematically shows an X-ray microscope.

DETAILED DESCRIPTION

An avalanche photodiode (APD) is a photodiode that uses the avalancheeffect to generate an electric current upon exposure to light. Theavalanche effect is a process where free charge carriers in a materialare subjected to strong acceleration by an electric field andsubsequently collide with other atoms of the material, thereby ionizingthem (impact ionization) and releasing additional charge carriers whichaccelerate and collide with further atoms, releasing more chargecarriers—a chain reaction. Impact ionization is a process in a materialby which one energetic charge carrier can lose energy by the creation ofother charge carriers. For example, in semiconductors, an electron (orhole) with enough kinetic energy can knock a bound electron out of itsbound state (in the valence band) and promote it to a state in theconduction band, creating an electron-hole pair.

An APD may work in the Geiger mode or the linear mode. When the APDworks in the Geiger mode, it may be called a single-photon avalanchediode (SPAD) (also known as a Geiger-mode APD or G-APD). A SPAD is anAPD working under a reverse bias above the breakdown voltage. Here theword “above” means that absolute value of the reverse bias is greaterthan the absolute value of the breakdown voltage. A SPAD may be used todetect low intensity light (e.g., down to a single photon) and to signalthe arrival times of the photons with a jitter of a few tens ofpicoseconds. A SPAD may be in a form of a p-n junction under a reversebias (i.e., the p-type region of the p-n junction is biased at a lowerelectric potential than the n-type region) above the breakdown voltageof the p-n junction. The breakdown voltage of a p-n junction is areverse bias, above which exponential increase in the electric currentin the p-n junction occurs. An APD working at a reverse bias below thebreakdown voltage is operating in the linear mode because the electriccurrent in the APD is proportional to the intensity of the lightincident on the APD.

FIG. 1 schematically shows the electric current in an APD as a function112 of the intensity of light incident on the APD when the APD is in thelinear mode, and a function 111 of the intensity of light incident onthe APD when the APD is in the Geiger mode (i.e., when the APD is aSPAD). In the Geiger mode, the current shows a very sharp increase withthe intensity of the light and then saturation. In the linear mode, thecurrent is essentially proportional to the intensity of the light.

FIG. 2A, FIG. 2B and FIG. 2C schematically show the operation of an APD,according to an embodiment. FIG. 2A shows that when a photon (e.g., anX-ray photon) is absorbed by an absorption region 210, multiple (100 to10000 for an X-ray photon) electron-hole pairs maybe generated. Theabsorption region 210 has a sufficient thickness and thus a sufficientabsorptance (e.g., >80% or >90%) for the incident photon. For soft X-rayphotons, the absorption region 210 may be a silicon layer with athickness of 10 microns or above. The electric field in the absorptionregion 210 is not high enough to cause avalanche effect in theabsorption region 210. FIG. 2B shows that the electrons and hole driftin opposite directions in the absorption region 210. FIG. 2C shows thatavalanche effect occurs in an amplification region 220 when theelectrons (or the holes) enter that amplification region 220, therebygenerating more electrons and holes. The electric field in theamplification region 220 is high enough to cause an avalanche of chargecarriers entering the amplification region 220 but not too high to makethe avalanche effect self-sustaining. A self-sustaining avalanche is anavalanche that persists after the external triggers disappear, such asphotons incident on the APD or charge carriers drifted into the APD. Theelectric field in the amplification region 220 may be a result of adoping profile in the amplification region 220. For example, theamplification region 220 may include a p-n junction or a heterojunctionthat has an electric field in its depletion zone. The threshold electricfield for the avalanche effect (i.e., the electric field above which theavalanche effect occurs and below which the avalanche effect does notoccur) is a property of the material of the amplification region 220.The amplification region 220 may be on one or two opposite sides of theabsorption region 210.

FIG. 3A schematically shows a cross section of an image sensor 300 basedon an array of APDs 350. Each of the APDs 350 may have an absorptionregion 310 and a first amplification region 320 as the example shown inFIG. 2A, FIG. 2B and FIG. 2C, as well as an electrode 301. At leastsome, or all, of the APDs 350 in the image sensor 300 may have theirabsorption regions 310 joined together. Namely, the image sensor 300 mayhave joined absorption regions 310 in a form of an absorption layer 311that is shared among at least some or all of the APDs 350. The firstamplification regions 320 of the APDs 350 are discrete regions. Namelythe first amplification regions 320 of the APDs 350 are not joinedtogether. The electrode 301 is electrically connected to the absorptionregion 310. The electrode 301 of at least some or all of the APDs 350may be joined together. In an embodiment, the electrode 301 may be acommon electrode shared by the absorption regions 310 of the APDs 350.The image sensor 300 may further include a heavily doped layer 302disposed on the absorption regions 310 opposite to the firstamplification region 320, and the electrode 301 may be on the heavilydoped layer 302. The electrode 301 of at least some or all of the APDs350 may be joined together. The heavily doped layer 302 of at least someor all of the APDs 350 may be joined together. In an embodiment, each ofthe APDs 350 further comprises a second amplification region between theabsorption region 310 and the electrode 301.

The image sensor 300 may further include electrodes 304 respectively inelectrical contact with the layer 313 of the APDs 350. The electrodes304 are configured to collect electric current flowing through the APDs350.

The image sensor 300 may further include a passivation material 303configured to passivate surfaces of the absorption regions 310 and thelayer 313 of the APDs 350 to reduce recombination at these surfaces.

In an embodiment, the absorption layer 311 may be in form of asemiconductor wafer such as a silicon wafer. The absorption regions 310may be an intrinsic semiconductor or very lightly doped semiconductor(e.g., <10¹² dopants/cm³, <10¹¹ dopants/cm³, <10¹⁰ dopants/cm³, <10⁹dopants/cm³), with a sufficient thickness and thus a sufficientabsorptance (e.g., >80% or >90%) for incident photons of interest (e.g.,X-ray photons). The first amplification regions 320 may have a junction315 formed by at least two layers 312 and 313. The junction 315 may be aheterojunction of a p-n junction. In an embodiment, the layer 312 is ap-type semiconductor (e.g., silicon) and the layer 313 is a heavilydoped n-type layer (e.g., silicon). The phrase “heavily doped” is not aterm of degree. A heavily doped semiconductor has its electricalconductivity comparable to metals and exhibits essentially linearpositive thermal coefficient. In a heavily doped semiconductor, thedopant energy levels are merged into an energy band. A heavily dopedsemiconductor is also called degenerate semiconductor. The layer 312 mayhave a doping level of 10¹³ to 10¹⁷ dopants/cm³. The layer 313 may havea doping level of 10¹⁸ dopants/cm³ or above. The layers 312 and 313 maybe formed by epitaxy growth, dopant implantation or dopant diffusion.The band structures and doping levels of the layers 312 and 313 can beselected such that the depletion zone electric field of the junction 315is greater than the threshold electric field for the avalanche effectfor electrons (or for holes) in the materials of the layers 312 and 313,but is not too high to cause self-sustaining avalanche. Namely, thedepletion zone electric field of the junction 315 should cause avalanchewhen there are incident photons in the absorption region 310 but theavalanche should cease without further incident photons in theabsorption region 310.

The junctions 315 of the APDs 350 should be discrete, i.e., the junction315 of one of the APDs 350 should not be joined with the junction 315 ofanother one of the APDs 350. Charge carriers amplified at one of thejunctions 315 of the APDs 350 should not be shared with another of thejunctions 315. The junction 315 of one of the APDs 350 may be separatedfrom the junction 315 of the neighboring APDs 350 by the material of theabsorption region wrapping around the junction, by the material of thelayer 312 or 313 wrapping around the junction, by an insulator materialwrapping around the junction, or by a guard ring of a dopedsemiconductor. As shown in FIG. 3A, the layer 312 of each of the APDs350 may be discrete, i.e., not joined with the layer 312 of another oneof the APDs 350; the layer 313 of each of the APDs 350 may be discrete,i.e., not joined with the layer 313 of another one of the APDs 350.

The electrode 301 may comprise silver nanoparticles 322 as shown in FIG.3B and FIG. 3C. The silver nanoparticles 322 may have a number densityabove an electrical percolation threshold of the silver nanoparticles322 in the electrode 301. The electrical percolation threshold is acritical number density of the silver nanoparticles 322, above which thesilver nanoparticles 322 of the electrode 301 may contact one anotherand form an electrically conductive path allowing charge carriers toflow through. In an embodiment, the silver nanoparticles 322 maycomprise silver nanowires, and the silver nanowires may form aconductive network as shown in FIG. 3B and FIG. 3C. The silvernanoparticles 322 may form electrical contact (e.g., Ohmic contact) withthe absorption region 310 or the heavily doped layer 302. The electrode301 may further comprise a conductive pad 324 in electrical contact witha portion of the silver nanoparticles 322. The silver nanoparticles 322may have various geometries, sizes, shapes or aspect ratios (e.g., ratioof the sizes of the silver nanoparticles in different dimensions). Forinstance, the silver nanowires in FIG. 3B and FIG. 3C may have lengthsof nanometers or micrometers, and diameters of nanometers to hundreds ofnanometers. The silver nanoparticles 322 may also be spheres or otheranisotropic structures besides nanowires, or may be a hybrid of variousshapes.

The electrode 301 may be a hybrid electrode, which further comprises acoating layer 326 on the silver nanoparticles 322, as shown in FIG. 3C.The coating layer 326 may comprise insulating materials such as heatresistant polymers, or conductive materials such as conducting polymers,indium tin oxide (ITO), graphene, silver, etc. The coating layer 326 mayimprove mechanical strength of the electrode 301 and help protect thesilver nanoparticles 322. The conductivity of the electrode 301 may bedetermined by intrinsic conductivity of the silver nanoparticles 322,the number density of the silver nanoparticles 322, geometry of thesilver nanoparticles 322, and the coating material, etc. In anembodiment, the conductivity of the electrode 301 may be comparable tothat of bulk silver. The electrode 301 may be transparent to light withwavelength in a variety of regions, such as X-ray region, visible regionand infrared region. For instance, the transmittance of the electrode301 in visible light region and infrared region may reach to 70%, 80%,90%, and above.

When a photon incidents on the image sensor 300, it may be absorbed bythe absorption region 310 of one of the APDs 350, and charge carriersmay be generated in the absorption region 310 as a result. One type(electrons or holes) of the charge carriers may drift toward the firstamplification region 320 of that one APD. When the one type of chargecarriers enters the first amplification region 320, the avalanche effectoccurs and causes amplification of the charge carriers. The amplifiedcharge carriers can be collected through the electrode 304 of that oneAPD 350, as an electric current. The other type of charge carriers(holes or electrons) generated in the absorption region 310, oramplified in the second amplification region if one exists, may flow tothe silver nanoparticles 322 and then be collected through theconductive pad 324. When that one APD 350 is in the linear mode, theelectric current is proportional to the number of incident photons inthe absorption region 310 per unit time (i.e., proportional to the lightintensity at that one APD). The electric currents at the APDs 350 may becompiled to represent a spatial intensity distribution of light, i.e.,an image. The amplified charge carriers may alternatively be collectedthrough the electrode 304 of that one APD 350, and the number of photonsmay be determined from the charge carriers (e.g., by using the temporalcharacteristics of the electric current).

FIG. 3D shows a variant of the image sensor 300, where the layers 312 ofsome or all of the APDs 350 are joined together. FIG. 3E shows a variantof the image sensor 300, where the junction 315 is surrounded by a guardring 316. The guard ring 316 may be an insulator material or a dopedsemiconductor. For example, when the layer 313 is heavily doped n-typesemiconductor, the guard ring 316 may be n-type semiconductor of thesame material as the layer 313 but not heavily doped. The guard ring 316may be present in the image sensor 300 shown in FIG. 3A or FIG. 3D. FIG.3F shows a variant of the image sensor 300, where the junction 315 hasan intrinsic semiconductor layer 317 sandwiched between the layer 312and 313. The intrinsic semiconductor layer 317 in each of the APDs 350may be discrete, i.e., not joined with other intrinsic semiconductorlayer 317 of another APD 350. The intrinsic semiconductor layers 317 ofsome or all of the APDs 350 may be joined together.

FIG. 4A schematically shows a detailed cross-sectional view of an imagesensor 400, according to an embodiment. The image sensor 400 may includea radiation absorption layer 410 configured to absorb an incidentradiation and generate electrical signals from incident radiation, andan electronics layer 420 (e.g., an ASIC) for processing or analyzing theelectrical signals generates in the radiation absorption layer 410.

The radiation absorption layer 410 may comprise a substrate 402, one ormore recesses 404 in the substrate 402, each of which having asemiconductor single crystal 406 in it, and an electrode 419A on the oneor more semiconductor single crystals 406. In an embodiment, at leastsome of the recesses 404 each have one and only one semiconductor singlecrystal 406, i.e., they each contain no other semiconductor materialexcept the one semiconductor single crystal 406. The substrate 402 maycomprise silicon, germanium, GaAs or a combination thereof. Each of thesemiconductor single crystals 406 may be a cadmium zinc telluride(CdZnTe) single crystal, a cadmium telluride (CdTe) single crystal, orany other suitable single crystals that can be used to absorb radiationparticles incident thereon and generate charge carriers. The electrode419A may be an embodiment of the electrode 301 as shown in FIG. 3B andFIG. 3C. The silver nanoparticles 422 of the electrode 419A may be inelectrical contact with the one or more single crystals 406. Theradiation absorption layer 410 may further comprise another electrode419B on a surface (e.g., an exposed surface, namely a surface not indirect physical contact with the substrate 402) of the semiconductorsingle crystals 406, and the electrode 419B may comprise discreteregions. Each of the semiconductor single crystals 406 may also be incontact with one or more discrete regions of the electrode 419B. Thesurface of the substrate 402 may be coextensive with the surface of eachof the semiconductor single crystals 406. In an embodiment, the surfaceof each of the semiconductor single crystals 406 may accommodate tens orhundreds of the discrete regions of the electrode 419B. The electrode419B may comprise a conducting material such as a metal (e.g., gold,copper, aluminum, platinum, etc.), or any other suitable conductingmaterials (e.g., a doped semiconductor). The electrodes 419A and 419Bmay be configured to collect the charge carriers (e.g., electrons andholes) generated in the semiconductor single crystal 406.

When the radiation hits the radiation absorption layer 410, thesemiconductor single crystals 406 may absorb the radiation particlesincident thereon and generate one or more charge carriers by a number ofmechanisms. A particle of the radiation may generate 10 to 100000 chargecarriers. The charge carriers may drift to the electrodes 419A and 419Bunder an electric field. The field may be an external electric field. Inan embodiment, one type of charge carriers may drift in directions suchthat the charge carriers generated by a single particle of the radiationare not substantially shared by two different discrete portions of theelectrode 419B (“not substantially shared” here means less than 2%, lessthan 0.5%, less than 0.1%, or less than 0.01% of these charge carriersflow to a different one of the discrete portions than the rest of thecharge carriers). The one type of charge carriers generated by aradiation particle incident around the footprint of one of thesediscrete portions of the electrode 419B are not substantially sharedwith another of these discrete portions of the electrode 419B. An areaaround a discrete portion of the electrode 419B may be considered as apixel associated with the discrete portion of the electrode 419B, wheresubstantially all (more than 98%, more than 99.5%, more than 99.9% ormore than 99.99% of) the one type of charge carriers generated by aparticle of the radiation incident therein flow to the discrete portionof the electrode 419B. Namely, less than 2%, less than 0.5%, less than0.1%, or less than 0.01% of these charge carriers flow beyond the pixelassociated with the one discrete portion of the electrode 419B. Theother type of charge carriers generated in the semiconductor singlecrystals 406 may flow to the silver nanoparticles 422 and then becollected through one or more conductive pads of the electrode 419A asin the examples shown in FIG. 3B and FIG. 3C.

The electronics layer 420 may include an electronic system 421configured to process electrical signals on the electrode 419B generatedfrom the charge carriers collected. The electronic system 421 mayinclude an analog circuitry such as a filter network, amplifiers,integrators, and comparators, or a digital circuitry such as amicroprocessor, and memory. The electronic system 421 may include one ormore ADCs. The electronic system 421 may include components shared bythe pixels or components dedicated to a single pixel. For example, theelectronic system 421 may include an amplifier dedicated to each pixeland a microprocessor shared among all the pixels. The electronic system421 may be electrically connected to the pixels by vias 431. Space amongthe vias 431 may be filled with a filler material 430, which mayincrease the mechanical stability of the connection of the electronicslayer 420 to the radiation absorption layer 410. Other bondingtechniques are possible to connect the electronic system 421 to thepixels without using vias.

FIG. 4B schematically shows a top view of the radiation absorption layer410 in FIG. 4A, according to an embodiment. Each of the recesses 404 mayhave a shape of a frustum, prism, pyramid, cuboid, cubic or cylinder.The recesses 404 may be arranged into an array such as a rectangulararray, a honeycomb array, a hexagonal array or any other suitable array.In example of FIG. 4B, the recesses 404 are arranged into a rectangulararray, and each of the recesses 404 has a pyramid shape. The recesses404 are shown in dashed line since they cannot be seen directly from thetop view.

FIG. 5 schematically illustrates a process of forming an electrode 501,according to an embodiment. The electrode 501 may function as theelectrode 301 in FIG. 3B, 3C or 419A in FIG. 4A.

In step 10, silver nanoparticles 522 is deposited onto a substrate 502.The substrate 502 may serve as the absorption region 310 or the heavilydoped layer 302 in FIG. 3A or the semiconductor single crystals 406 inFIG. 4A. The silver nanoparticles 522 may serve as the silvernanoparticles 322 in FIG. 3A or 422 in FIG. 4A. The silver nanoparticles522 may be deposited onto the substrate 502 by various techniques,including pressure dispensing, jet dispensing, spin coating,roll-to-roll coating, screen printing, inject printing, off-set printingand micro-contact printing, etc. For instance, silver nanoparticles 522may be first uniformed dispersed in a polar or non-polar solvent (suchas tetradecane, alcohol or water) with a suitable solid content (e.g.,weight percentage around 30%-90%) to form a silver nanoparticle ink. Thesilver nanoparticle ink may be applied onto the substrate 502 bypressure dispensing with a dispensing system. A sintering or curing stepmay be followed to sinter the junctions (i.e., contact areas) among thesilver nanoparticles 522 to reduce the resistance of the electrode 501and to help removing the dispersing solvent. Sintering or curing may bedone by annealing the electrode 501 under a suitable temperature (e.g.,a temperature ranging from 100° C. to 700° C.) for a certain timeduration (e.g., 10 minutes, 60 minutes, etc.). For instance, theelectrode 501 may be cured below 200° C. when the substrate 502 is thesemiconductor single crystals 406 such as cadmium zinc telluride(CdZnTe) single crystal or cadmium telluride (CdTe) single crystal.Other sintering or curing methods may also be used, such as laserannealing, mechanical pressing, plasmon-welding and local chemicalwielding, etc.

In step 20, a conductive pad 524 is formed on the substrate 502, theconductive pad 524 being in electrical contact with a portion of thesilver nanoparticles 522. The conductive pad 524 may be formed bydepositing a conductive material (e.g., a metal such as Pt, Au or In, orany other suitable conducting materials) onto the substrate 502 by asuitable technique such as physical vapor deposition, chemical vapordeposition, spin coating, sputtering, etc. In the example of the step 20of FIG. 5, the conductive pad 524 may comprise one or more regions onthe edge of the substrate 502.

In an optional step 30, a coating layer 526 is coated onto the silvernanoparticles 522. The coating layer 526 may comprise insulatingmaterials such as heat resistant polymers, or conductive materials suchas conducting polymers, Indium tin oxide (ITO), graphene, silver, etc.Various coating methods may be applied depending on the choice ofcoating material. For instance, the coating layer 526 may comprise heatresistant polymers or conducting polymers, and may be formed by firstcoating or dispensing a polymer or monomer solution on to the silvernanoparticles 522, and then curing the polymer or monomer solution.

FIG. 6 schematically shows a system comprising an apparatus 600 beingthe image sensor 300 or 400 described herein. The system comprises anX-ray source 601. X-ray emitted from the X-ray source 601 penetrates anobject 610 (e.g., diamonds, tissue samples, a human body part such asbreast), is attenuated by different degrees by the internal structuresof the object 610, and is projected to the apparatus 600. The apparatus600 forms an image by detecting the intensity distribution of the X-ray.The system may be used for medical imaging such as chest X-rayradiography, abdominal X-ray radiography, dental X-ray radiography,mammography, etc. The system may be used for industrial CT, such asdiamond defect detection, scanning a tree to visualize year periodicityand cell structure, scanning building material like concrete afterloading, etc.

FIG. 7 schematically shows an X-ray computed tomography (X-ray CT)system. The X-ray CT system uses computer-processed X-rays to producetomographic images (virtual “slices”) of specific areas of a scannedobject. The tomographic images may be used for diagnostic andtherapeutic purposes in various medical disciplines, or for flawdetection, failure analysis, metrology, assembly analysis and reverseengineering. The X-ray CT system comprises an apparatus 700 being theimage sensor 300 or 400 described herein and an X-ray source 701. Theapparatus 700 and the X-ray source 701 may be configured to rotatesynchronously along one or more circular or spiral paths.

FIG. 8 schematically shows an X-ray microscope or X-ray micro CT 800.The X-ray microscope or X-ray micro CT 800 may include an X-ray source801, focusing optics 804, and an apparatus 803 being the image sensor300 or 400 described herein, for detecting an X-ray image of a sample802.

While various aspects and embodiments have been disclosed herein, otheraspects and embodiments will be apparent to those skilled in the art.The various aspects and embodiments disclosed herein are for purposes ofillustration and are not intended to be limiting, with the true scopeand spirit being indicated by the following claims.

What is claimed is:
 1. An apparatus comprising: an array of avalanchephotodiodes (APDs), each of the APDs comprising an absorption region, anelectrode and a first amplification region; wherein the absorptionregion is configured to generate charge carriers from a photon absorbedby the absorption region; wherein the electrode comprises silvernanoparticles and is electrically connected to the absorption region;wherein the first amplification region comprises a junction with anelectric field in the junction; wherein the electric field is at a valuesufficient to cause an avalanche of charge carriers entering the firstamplification region, but not sufficient to make the avalancheself-sustaining; wherein the junctions of the APDs are discrete; whereinthe silver nanoparticles comprise silver nanowires.
 2. The apparatus ofclaim 1, wherein each of the APDs further comprises a secondamplification region between the absorption region and the electrode,wherein the first amplification region and the second amplificationregion are on opposite sides of the absorption region.
 3. The apparatusof claim 1, wherein a number density of the silver nanoparticles isabove an electrical percolation threshold of the silver nanoparticles.4. The apparatus of claim 1, wherein the electrode further comprises aconductive pad in electrical contact with a portion of the silvernanoparticles.
 5. The apparatus of claim 1, wherein the electrode is acommon electrode shared by the absorption regions of the array of APDs.6. The apparatus of claim 1, wherein the electrode further comprises acoating layer on the silver nanoparticles.
 7. The apparatus of claim 1,wherein the photon is a soft X-ray photon.
 8. The apparatus of claim 1,wherein the absorption region has a thickness of 10 microns or above. 9.The apparatus of claim 1, wherein the absorption region comprisessilicon.
 10. The apparatus of claim 1, wherein an electric field in theabsorption region is not high enough to cause avalanche effect in theabsorption region.
 11. The apparatus of claim 1, wherein the absorptionregion is an intrinsic semiconductor or a semiconductor with a dopinglevel less than 10¹² dopants/cm³.
 12. The apparatus of claim 1, whereinthe absorption regions of at least some of the APDs are joined together.13. The apparatus of claim 1, wherein the first amplification regions ofthe APDs are discrete.
 14. The apparatus of claim 1, wherein thejunction is a p-n junction or a heterojunction.
 15. The apparatus ofclaim 1, wherein the junction comprises a first layer and a secondlayer, wherein the first layer is a doped semiconductor and the secondlayer is a heavily doped semiconductor.
 16. The apparatus of claim 15,wherein the first layer has a doping level of 10¹³ to 10¹⁷ dopants/cm³.17. The apparatus of claim 15, wherein the first layers of least some ofthe APDs are joined together.
 18. The apparatus of claim 15, wherein thejunction is separated from a junction of a neighboring APD by a materialof the absorption region, a material of the first or second layer, aninsulator material, or a guard ring of a doped semiconductor.
 19. Theapparatus of claim 15, wherein the junction is separated from a junctionof a neighboring APD by a guard ring of a doped semiconductor; whereinthe doped semiconductor has a same doping type as the second layer andthe guard ring is not heavily doped.
 20. The apparatus of claim 15,wherein the junction further comprises a third layer sandwiched betweenthe first and second layers; wherein the third layer comprises anintrinsic semiconductor.
 21. The apparatus of claim 20, wherein thethird layers of at least some of the APDs are joined together.
 22. Asystem comprising the apparatus of claim 1 and an X-ray source, whereinthe system is configured such that the apparatus forms an image of anobject using X-ray from the X-ray source that penetrated the object. 23.An apparatus comprising: a substrate; a semiconductor single crystal ina recess in the substrate; an electrode on the semiconductor singlecrystal; wherein the semiconductor single crystal is configured toabsorb radiation particles incident thereon and to generate chargecarriers; wherein the electrode comprises silver nanoparticles and iselectrically connected to the semiconductor single crystal.
 24. Theapparatus of claim 23, wherein the silver nanoparticles are silvernanowires.
 25. The apparatus of claim 23, wherein a number density ofthe silver nanoparticles is above an electrical percolation threshold ofthe silver nanoparticles.
 26. The apparatus of claim 23, wherein theelectrode further comprises a conductive pad in electrical contact witha portion of the silver nanoparticles.
 27. The apparatus of claim 23,wherein the electrode further comprises a coating layer on the silvernanoparticles.
 28. The apparatus of claim 23, wherein the semiconductorsingle crystal is a CdZnTe single crystal or a CdTe single crystal. 29.The apparatus of claim 23, wherein the substrate comprises silicon,germanium, GaAs or a combination thereof.
 30. The apparatus of claim 23,wherein a surface of the semiconductor single crystal and a surface ofthe substrate are coextensive.
 31. The apparatus of claim 23, furthercomprising another electrode in electrical contact with thesemiconductor single crystal; an electronics layer bonded to thesubstrate, the electronics layer comprising an electronic systemconfigured to process an electrical signal generated from the chargecarriers collected by the other electrode.
 32. The apparatus of claim31, wherein the electronic system comprises a voltage comparatorconfigured to compare a voltage of the electrode to a first threshold; acounter configured to register a number of radiation particles absorbedby the substrate; a controller; a voltmeter; wherein the controller isconfigured to start a time delay from a time at which the voltagecomparator determines that an absolute value of the voltage equals orexceeds an absolute value of the first threshold; wherein the controlleris configured to cause the voltmeter to measure the voltage uponexpiration of the time delay; wherein the controller is configured todetermine a number of radiation particles by dividing the voltagemeasured by the voltmeter by a voltage that a single radiation particlewould have caused on the other electrode; wherein the controller isconfigured to cause the number registered by the counter to increase bythe number of radiation particles.
 33. A system comprising the apparatusof claim 23 and an X-ray source, wherein the system is configured suchthat the apparatus forms an image of an object using X-ray from theX-ray source that penetrated the object.